1. Field of the Invention
In general, the present invention relates to measurement devices used to properly fit eyeglasses. More particularly, the present invention relates to instruments used to determine the rear vertex distance between a lens and the eye.
2. Prior Art Description
When eyeglasses are created for a particular person, several measurements must be made. Many of those measurements depend upon the center of the pupils in relation to the lenses used in the eyeglasses. Eyeglasses can be either framed or frameless. Framed eyeglasses have lenses that are mounted on a frame. Frameless glasses have lenses that are only secured by a nosepiece and the ear handles of the eyeglasses.
When the wearer is at the opticians buying a new pair of eyeglasses, certain centering data has to be recorded in order to properly fit the lenses. This data is necessary for the grinding of the lenses and for fitting the lenses onto a frame. In order to ensure that the lenses correspond to the wearer's individual requirements, the manufacturers need to know a point on the lens directly in front of the pupil when the wearer is looking straight ahead and is said to have a natural posture and head position. When it comes to measuring the height of the middle of the pupil in relation to the lowest part of the spectacle frame, or in the case of frameless glasses in relation to the lowest part of the lens, the normal approach is that the optician will stand in front of the wearer and ask him or her to look straight ahead. At the same time, the wearer will be requested to take on a natural posture and head position. The optician will then try to sight the middle of the pupil and mark it on the lens using a felt marker. In the case of a new pair of spectacles, an optician will mark the demo glass.
Likewise, the optician needs to measure the rear vertex distance between the eyeglass lenses and the wearer's eyes. Each lens has a rear surface that faces the wearer. The rear vertex distance is the distance between the vertex of the cornea and the rear surface of the lens. Traditionally, this measurement is obtained by viewing or taking a picture of the wearer from the side. The measurement is then obtained using a ruler or by measuring the distance from the photograph.
There are a number of disadvantages associated with the traditional manner of measuring the center point on a lens and determining the rear vertex distance from this center point. If the wearer were to take on a body posture and head position that is different to his/her natural one, a wrong measurement can be recorded. For example, if the wearer tilts his/her head further back than normal, this causes him to look through a lower part of the lens in relation to the lowest part of the eyeglasses. The optician would then incorrectly sight the position of the middle of the pupil because of the wearer's unnatural posture and head position. If the lenses of the eyeglasses are made using this incorrect measurement, the eyeglasses would be incorrect. Furthermore, it often happens that a person will move his/her head between when the center of the lens is measured and when the rear vertex distance is measured. If the head moves between these measurements, the measurements will be adversely effected and the eyeglasses will be made incorrectly. Furthermore, if the measurement of the vertex distance is taken from the side, there will be parallax that makes the measurement inaccurate unless the patient's head is held firmly at 90° to the optician or the camera.
It has been observed that a person often holds his/her head in an unnatural position when being fitted for eyeglasses and/or while being photographed. When the optician stands near a person to take a pupil measurement or a photograph, that person often assumes a posture and head position that is different to his natural one. For example, a person may stand far more erect that he normally would. Alternatively, a person may tilt his head further back or forward than normal. The incorrect measurements may render the manufactured eyeglasses unusable, with the result that the optician has to redo the measurements and replace the lenses.
In the prior art, measurement devices have been developed that are used in the proper fitting of eyeglasses. For instance U.S. Pat. No. 4,505,043 to Sztuka, entitled Height Measurement Gage For Multifocal Lenses, a measurement device is shown that attaches to the frame of a pair of eyeglasses. The device records the position of the pupil relative the eyeglass frame. This eliminates the need to mark the lens with a felt marker. However, the device is only suitable with eyeglasses having specific angles of inclination. Furthermore, the device does not eliminate errors caused by taking measurements while a person's head is in an unnatural position. The device also requires that a separate side picture be taken in order to measure the rear vertex distance.
A need therefore exists for a device that can be used to accurately measure the position of a person's pupil in relation to a pair of eyeglasses, without having a person move his/her head from a natural position and posture. A need also exists for a system and method of measuring the rear vertex distance for a set of eyeglasses using an accurate center point and not requiring a separate side photograph for measurement. These needs are met by the present invention as described and claimed below.